An exposure apparatus which transfers the pattern of a master such as a reticle onto a substrate such as a wafer or glass plate, coated with a photosensitive agent, is used to manufacture a device such as a semiconductor device or liquid crystal display device by photolithography.
Exposure apparatuses include a step & repeat exposure apparatus (a so-called stepper), a step & scan exposure apparatus (a so-called scanner or scanning exposure apparatus), and the like. The step & repeat exposure apparatus transfers a reticle pattern onto each shot region on a wafer while holding a wafer stage on which the wafer is mounted still. On the other hand, the step & scan exposure apparatus scans a reticle and wafer relative to a projection optical system while projecting part of a reticle pattern onto the wafer through the projection optical system, thereby transferring the reticle pattern onto each shot region on the wafer.
Operation of an exposure apparatus can be classified into normal exposure which transfers a reticle pattern onto a wafer and measurement exposure which measures the illuminance uniformity and the like on an exposure field.
FIG. 9 is a view showing the schematic arrangement of a conventional exposure apparatus. Note that an exposure apparatus shown in FIG. 9 can be implemented either as a stepper or as a scanner. Referring to FIG. 9, an excimer laser 1 serving as a pulse light source emits pulsed illumination light. Illumination light from the excimer laser 1 is shaped into a parallel beam having a desired sectional shape by a beam shaping optical system 2. Linearly polarized illumination light having passed through the beam shaping optical system 2 is converted into circularly polarized illumination light by a quarter-wave plate 3. The circularly polarized illumination light is reflected by a reflecting mirror 4 and is guided to a fly-eye lens 5. Many light source images are formed on the exit surface of the fly-eye lens 5, thereby making the illuminance distribution of the illumination light uniform.
A beam splitter 6 transmits most of illumination light having passed through the fly-eye lens 5 to input it to a lens group 7 and reflects the remaining part of illumination light to input it to an integrated exposure amount sensor 15. The lens group 7 illuminates a reticle (master) 9 with a uniform illuminance distribution. In this example, a reflecting mirror 8 is arranged in the lens group 7.
A pattern of the reticle 9 illuminated with illumination light is projected and transferred onto a wafer (substrate) 11 via a projection optical system 10. The wafer 11 is mounted on a wafer stage (substrate stage) 12 and is moved or aligned by the wafer stage 12. This makes it possible to transfer a pattern onto a target shot region on the wafer. An illuminance uniformity sensor 13 is arranged on the wafer stage 12 and is used only in measurement exposure.
Light reflected by the beam splitter 6 is condensed on the integrated exposure amount sensor 15 by a condenser lens 14. The integrated exposure amount sensor 15 can be used not only in indirect monitoring of the exposure amount of the wafer during normal exposure but also in measurement exposure. A main control system 16 controls the excimer laser 1 on the basis of output signals from the illuminance uniformity sensor 13 and integrated exposure amount sensor 15, thereby controlling exposure of the wafer.
As is commonly known, each photoelectric sensor (the illuminance uniformity sensor 13 or integrated exposure amount sensor 15) used for an exposure apparatus changes in sensitivity due to a change in temperature of the light-receiving surface, and this affects the exposure accuracy.
For example, in exposure amount control, the excimer laser 1 is controlled on the basis of an output signal from the integrated exposure amount sensor 15, thereby adjusting the exposure amount of the wafer. If the integrated exposure amount sensor 15 changes in sensitivity due to a change in temperature, an actual exposure amount of the wafer cannot be detected or estimated. Consequently, the exposure amount control accuracy decreases.
In addition, if the illuminance uniformity sensor 13 changes in sensitivity in measurement of the illuminance uniformity on an exposure field, a measurement error may occur. For example, a scanner can utilize an output signal from the illuminance uniformity sensor 13 to correct illuminance nonuniformity using a slit or the like. Hence, a change in sensitivity of the illuminance uniformity sensor 13 can result in illuminance nonuniformity in normal exposure.
FIG. 2 is a view showing the relationship among an exposure region (a region to be exposed), an illumination region, and the illuminance uniformity sensor 13. In normal exposure, the wafer stage 12 is driven in a scanning direction such that an exposure region 18 passes by immediately below a slit illumination region 17 at a constant velocity. On the other hand, in measurement exposure, the wafer stage 12 is driven such that the illuminance uniformity sensor 13 is located immediately below the slit illumination region 17, thereby measuring the illuminance uniformity.
For example, to measure the illuminance uniformity in a slit direction (the longitudinal direction of a slit), the illuminance uniformity sensor 13 starts measurement from a point Y1 in FIG. 2 and measures the illuminance uniformity of a region extending to a point Y2 at regular intervals. At this time, the quantity of light (integrated light quantity) with which the illuminance uniformity sensor 13 is irradiated increases as the illuminance uniformity sensor 13 moves from a start position (Y1) to an end position (Y2). This causes a rise in temperature on the light-receiving surface of the illuminance uniformity sensor 13, thus resulting in a change in sensitivity of the illuminance uniformity sensor 13. FIG. 3 is a graph showing an example of a change in sensitivity of the illuminance uniformity sensor 13 which may occur while the illuminance uniformity sensor 13 is moving in the slit direction. A change in sensitivity of the illuminance uniformity sensor 13, as shown in FIG. 3, decreases the measurement accuracy of illuminance or illuminance uniformity.
In recent years, the oscillation frequency of a pulse light source is increasing along with an increase in throughput of an exposure apparatus. For this reason, a change in sensitivity due to a change in temperature of a photoelectric sensor may greatly affect the exposure accuracy.
As a prior art reference that is related to such a problem, there is available Japanese Patent Laid-Open No. 9-22120. An apparatus disclosed in this reference comprises a photoelectric sensor which measures the light quantity of a light beam as part of illumination light and a temperature detection means for detecting the temperature on the light-receiving surface of the photoelectric sensor. The apparatus corrects a change in sensitivity caused by a change in temperature of the photoelectric sensor on the basis of the temperature detected by the temperature detection means. The above reference also discloses an apparatus which comprises a photoelectric sensor which measures the light quantity of a light beam as part of illumination light, a temperature detection means for detecting the temperature on the light-receiving surface of the photoelectric sensor, and a temperature control means for controlling the temperature on the light-receiving surface of the photoelectric sensor. The apparatus controls the temperature control means on the basis of a value detected by the temperature detection means, stabilizes the temperature on the light-receiving surface of the photoelectric sensor, and keeps constant a change in sensitivity caused by a change in temperature.
However, it is difficult for the apparatuses disclosed in Japanese Patent Laid-Open No. 9-22120 to accurately correct an effect caused by a change in sensitivity due to a change in temperature. More specifically, even if the temperature detection means is arranged in the vicinity of the light-receiving surface of the photoelectric sensor, a slight difference in temperature occurs between the photoelectric sensor and the temperature detection means because they are separately provided. This disables accurate detection of the temperature on the light-receiving surface of the photoelectric sensor and accurate stabilization of the sensitivity of the photoelectric sensor. Accordingly, it is difficult to detect or estimate the accurate integrated exposure amount on a wafer.
The provision of a temperature detection means and temperature control means, as in the apparatuses disclosed in the patent publication, increases the complexity of the arrangement of a wafer stage and reduces the temperature stability. This may pose problems such as a decrease in performance of, e.g., a wafer stage and an increase in cost of the apparatus.